Method for supporting a slow wave circuit via an array of dielectric posts

ABSTRACT

A slow wave circuit is supported within a surrounding metallic barrel portion of a traveling wave tube via the intermediary of an array of dielectric posts each of which is brazed at one end to the slow wave circuit and at the other end to the barrel. During fabrication of the tube, the post structures are first bonded to a metallic strip which in turn is bonded to the inside of the barrel. The strip preferably has a coefficient of thermal expansion equal to that of the dielectric posts.

United States Patent [15] 3,691,630 Burgess et al. 51 Sept. 19, 1972 METHOD FOR SUPPORTING A SLOW [56] References Cited WAVE CIRCUIT VIA AN ARRAY OF DIELECTRIC POSTS UNITED STATES PATENTS 3 496 617 2/1970 Cook et a1. ..29/600 X [72] Inventors. James E. Burgess, 205 Silvia Court,

Los Altos Cant 94022; Louis 3,505,730 4/1970 Nelson ..29/600 g:::; :3 g:i; 322s Primary Examiner-John F. Campbell L Grab m 2 uh C t Assistant Examiner-Robert W. Church a a our Attorney-Stanley Z. Cole and Gerald L. Moore Mllpltas, Calif. 95035, Allan W. Scott, 1272 Windimer, Los Altos, Calif. 94022 [57] ABSTRACT A slow wave circuit is supported within a surrounding [22] 1969 metallic barrel portion of a traveling wave; tube via the [21] Appl. No.: 883,733 intermediary of an array of dielectric posts each of which is brazed at one end to the slow wave circuit and at the other end to the barrel. During fabrication [52] g h gf of the tube, the post structures are first bonded to a [51] Int Cl map 1 0 metallic strip which in turn is bonded to the inside of o e e e e e e e e e e e e e e e e e e e e e e i e e e e l. ff f 581 Field of Search ..29/600, 601, 603,527,423, 6 barre The smp preferably has a 0 thermal expansion equal to that of the dielectric posts.

I 6 Claims, 12 Drawing Figures BRAZE POSTS l0 WTHERMALLY MATCH ED STRIP AND ETCH P'A'TENTETJsEP 1 9 m2 SHEET 1 OF 2 /SLOT DIELECTRICROD MMETALLIZE SLOTTED ROD BRAZE METALLIZED ROD TO HELIX SLOT Tl-ROUGH ROD TO FORM POSTS BRAZE POSTSTO -"THERMALLY MATCHED STRIP AND ETCH BRAZE HELIX SUBASSEMBLY INTO TUBE BODY I PATENTEDsEP 19 I972 SHEET 2 OF 2 BRAZE POST TO HELIX TO FORM SUBASSEMBLY BRAZE HELIX SUBASSEMBLY INTO TUBE BODY AS SHOWN IN FIG. 7

T S 0 P F 0 OF SLOTTED ROD TO METALLIC STRIP D 0 R R H 0 N 0 R H TR mm Sm '-SLOT DIELECTRIC ROD \METALLIZE BOTH SIDES FBRAZE SLOTTED ROD BY L Hm ATTORNEY METHOD FOR SUPPORTING A SLOW WAVE CIRCUIT VIA AN ARRAY OF DIELECTRIC POSTS DESCRIPTION OF THE PRIOR ART Heretofore, slow wave circuits have been supported within the barrel portion of traveling wave tubes via the intermediary of a plurality ,of dielectric rods which were brazed on one side to the slow wave circuit and on the opposite side to the inside wall of the barrel. In some of the prior art tubes, the support rods were transversely slotted to'form dielectric support fingers such that the dielectric material of the support structure was removed between adjacent periodic elements of the slow wave circuit to reduce undesired capacitive loading of the circuit to improve the interaction impedance of the circuit.

In the prior art structures, the thermal coefficient of expansion of the dielectric support structure, as of alumina or beryllia, was matched to the thermal coefficient of the tube body by making the tube body of a material or in a manner such that it had a coefficient of expansion equal to that of the ceramic support in order to prevent fracture of the dielectric support structure due to thermally produced stresses. In one case, a tube body was made of a porous tungsten matrix infiltrated with copper such that the composite aggregate material had a coefficient of expansion equal to that of the ceramic. The problem with this arrangement was that the tube body, to avoid gas leaks, had to be made excessively thick and brazing to the composite material was difficult.

In other cases, the tube body was made by wrapping and brazing together successive turns of a ribbon shaped metallic member so that when the space between turns of the tape were filled with brazing material the composite axially expansion would match that of the ceramic support structure. This composite structure was difficult to make in a vacuum tight manner.

In still other cases, the envelop of the tube has been constructed of alternate cylinders of molybdenum and monel with the axially dimensions of the cylinders being proportioned so that the composite linear expansion of the composite tube matched that of the ceramic support structure. Such a composite structure is difficult to fabricate and is to be avoided if possible.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of improved method and apparatus for supporting a slow wave circuit within a metallic barrel structure.

One feature of the present invention is the provision of an array of dielectric support posts interposed between the slow wave circuit and the surrounding metallic envelope with individual ones of the support posts being directed transversely of the slow wave circuit and being bonded at opposite ends to the slow wave circuit and to the envelope, respectively, whereby the expansion coefficient of the barrel can be substantially different than the expansion coefficient of the dielectric support posts to facilitate fabrication of the tube.

Another feature of the present invention is the same as the preceding feature wherein the barrel shaped envelope includes a metallic strip portion extending longitudinally of the slow wave circuit in registration with the outer ends of the support posts, such metallic strip being bonded at one side to the barrel and being bonded on the other side to the support posts, and such strip having a coefficient of thermal expansion substantially equal to that of the dielectric support posts to facilitate construction of the tube.

Another feature of the present invention is a method for fabricating a post supported slow wave circuit of the type described in the first feature wherein an elongated dielectric support structure is bonded to a metallic member and then transversely slotted to define an array of dielectric support posts bonded at the roots to the metallic member and then bonding the free end portions of the array of support posts to the slow wave circuit and affixing the metallic member to the body portion of the slow wave tube.

Another feature of the present invention is an alternative method for fabricating a slow wave circuit of the first feature wherein a dielectric support structure is bonded to the slow wave circuit and then transversely slotted to define an array of dielectric support posts ex tending transversely from the slow wave circuit to free end portions and then bonding the free end portions of the array of support posts to a metallic structure ex tending lengthwise of the slow wave circuit.

Other features and advantages of the present invention will become apparent upon perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are perspective fragmentary views of a dielectric support structure depicting successive steps in a method for fabricating a slow wave tube incorporating features of the present invention,

FIGS. 3-6 are perspective fragmentary vie'ws depicting a dielectric support affixed to adjacent turns of a helix slow wave circuit and depicting successive steps in the method for fabricating the posts supported slow wave circuit incorporating features of the present invention,

FIG. is a transverse sectional view of a slow wave tube incorporating features of the present invention,

FIGS. 8 and 9 are fragmentary perspective views of a dielectric support structure and depicting the first and second steps in an alternative method for fabricating a slow wave circuit incorporating features of the present invention,

FIGS. 10 and 11 are fragmentary perspective views of a helix support structure and depicting the third and fourth steps, respectively, in a method for fabricating a post supported slow wave circuit incorporating features of the present invention, and

FIG. 12 is a fragmentary perspective view of a post supported helix incorporating features of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1-7 there is shown the method for fabricating the post supported slow wave circuitshown in cross section in FIG. 7. More particularly, in step 1, the dielectric support rod 1, as of alumina or beryllia ceramic, is provided with a plurality of transverse slots 2 passing only partially through the rod 1 to define an array of support posts 3 projecting from a spine portion 4 defined by the unslotted portion of the rod I.

In step 2, the free end portions of the ceramic support posts 3 are metallized with a suitable metallizing compound, such as a conventional molybdenum-manganese composition, in the conventional manner.

In step 3, the free end portions of the support posts 3 are brazed to adjacent turns of a helix slow wave circuit 5 via any one of a number of conventional brazing alloys interposed between the circuit 5 and the metallized support posts 3. A plurality of such slotted and metallized rods 1 are brazed at intervals about the periphery of the helix, such as depicted in FIG. 7.

In step 4, the spine portion 4 of the support rods 1, as brazed to the slow wave circuit 5, are transversely slotted in registration with the previous slots 2 to form an array of support posts 3 which are brazed at one end to adjacent turns of the helix slow wave circuit 5 and which are free at their outer ends.

In step 5, the free end portions of the support posts 3 are metallized by any one of number of conventional metallizing compositions, such as the aforementioned molybdenum-manganese composition.

In step 6, the metallized free end portions of the support posts 3 are brazed to a metallic strip 6, as of 0.0 I 5 inch thick metal having a temperature coefficient of thermal expansion substantially equal to that of the ceramic posts 3. A particularly suitable metallic strip 6 is one made of a composition of tungsten and copper marketed under the trademark Elkonite and formed by a porous tungsten matrix having the pores thereof infiltrated with copper. After the slow wave circuit is brazed to the strip 6 via the ceramic support posts 3, the composite circuit is chemically etched to remove the excess brazing material from the support posts 3. Any one of a number of commercially available acid etches may be employed for this purpose.

In step 7, the composite slow wave circuit 5 with the brazed support posts 3 and longitudinally directed strips 6 are brazed into a cylindrical barrel portion 7 of the body of a conventional traveling wave tube. More particularly, the barrel 7 may be made of any one of a number of conventional envelope materials such as iron, copper, or a composite barrel formed by a succession of alternate iron and cupro nickel disks to form a composite envelope having an integral magnetic pole structure for periodic focusing of the beam of the traveling wave tube. The inside wall of the barrel 7 includes a plurality of longitudinally directed recesses at 8 having a depth substantially equal to the thickness of the metallic strip 6. The strip 6 is then brazed to the inside wall of the recess 8 via any one of a number of conventional brazing alloys having a melting point below the melting point of the braze material employed to braze the support posts 3 to the circuit 5. The advantage of the resultant post supported slow wave circuit, as shown in FIG. 7, is that the body 7 doesnt have to have a coefficient of thermal expansion which matches that of the support posts 3 or the circuit 5. Tubes employing the manufacturing method and structure depicted in FIGS. 1-7 have successfully operated at l kilowatt CW output power at X band.

Referring now to FIGS. 8-12 there is depicted an alternative method for fabricating the posts supported slow wave circuit of FIG. 7. In step I, the dielectric support rod 1, as previously described with regard to FIG. 1, is transversely slotted at 2 to form an array of support posts 3 carried from the spine portion 4.

In step 2, opposite sides of the slotted dielectric rod 1 are metallized at 11 by any one of a number of conventional metallizing materials, such as the conventional molybdenum-manganese composition.

In step 3, one side of the rod 1 is brazed to the metallic strip 6, as previously described with regard to FIG. 6, by means of a suitable brazing alloy.

In step 4, the spine portion 4 is slotted through in registration with the previous slots 2 to form a plurality of dielectric support posts 3 bonded at one end to the metallic strip 6 and metallized at their free ends 11.

In step 5, the metallized free ends 11 are brazed to adjacent turns of a helix 5. As in the previous example described above with regard to FIG. 6, a plurality of such strip supported post 3 arrays are brazed at intervals about the periphery of the slow wave circuit, such as at intervals, as shown in FIG. 7. After the support posts 3 have been brazed to the helix 5, the subassembly is chemically etched, in the manner as previously described with regard to FIG. 6 to remove the excess braze material from the support posts 3.

In step 6, the subassembly is brazed into the tube body 7 as previously described with regard to FIG. 7 and step 7 of the aforedescribed method, to form a stub supported slow wave circuit. The advantage in use of the method of FIGS. 8-12 is that it has one less step thanthe method previously described with regard to FIGS. 1-7 and, in addition, has the advantage that the support posts 3 are formed on the metallic strip 6 as contrasted with being brazed after formation to the metallic strip 6. This facilitates maintaining the proper spacing and jigging of the posts 3 as they are brazed to the support strip 6.

Although the methods of the present invention have been depicted as employed for supporting a helix, they are equally applicable to supports for other types of slow wave circuits such as ring and bar, contrawound helices, bifilar helices, double ring and bar, as well as many other types of circuits.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention can be made without departing from the scope thereof it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a method for fabricating a dielectric supported slow wave circuit the steps of, bonding a plurality of dielectric structures to respective ones of a plurality of metallic support members, transversely slotting each of said dielectric structures to define on each said structure an array of dielectric support posts bonded at their root portions to the respective metallic support member and extending therefrom to free end portions, bonding the free end portions of the array of support posts of each said structure to the slow wave circuit, and affixing each of the metallic support members to a body portion of a slow wave tube.

2. The method of claim 1 wherein the step of bonding the dielectric structures to the respective metallic members includes the step of metallizing opposite sides of each of the dielectric structures and bonding one of the metallized sides of each of the dielectric structures to one of the respective metallic members.

3. The method of claim 1 wherein the step of affixing each of the metallic members to the body portion comprises the step of bonding each of the metallic members to the body portion.

4. In a method for fabricating a dielectric post supported slow wave circuit the steps of, bonding a plurality of dielectric support structures to the slow wave circuit for supporting the slow wave circuit, transversely slotting each of the dielectric support structures to define a plurality of arrays of dielectric support posts bonded at one end to the slow wave circuit and extending away transversely therefrom to free end portions, and bonding the free end portions of each of the arrays of dielectric support posts to a separate metallic support structure extending lengthwise of the slow wave circuit.

5. The method of claim 4 wherein each of said metallie support structures is a metallic strip and including the step of bonding each of the elongated metallic strips to the body portion of the slow wave tube structure.

6. In a method for fabricating a dielectric supported slow wave circuit, the steps of, slotting an elongated dielectric support structure transversely to a longitudinal side thereof to form a series of posts interconnected by spline portions, bonding said side of the dielectric support structure to an elongated metallic support member, removing the spline portions from the dielectric support structure to define separate ones of said posts bonded at the root portions thereof to the metallic support member and extending in parallel relationship to free end portions terminating along a line parallel to said longitudinal side, while the free end portions remain along said line bonding the free end portions of said separate posts to spaced portions of the slow wave circuit, and bonding the metallic support member to a body portion of a slow wave tube to support the slow wave circuit in the slow wave tube. 

1. In a method for fabricating a dielectric supported slow wave circuit the steps of, bonding a plurality of dielectric structures to respective ones of a plurality of metallic support members, transversely slotting each of said dielectric structures to define on each said structure an array of dielectric support posts bonded at their root portions to the respective metallic support member and extending therefrom to free end portions, bonding the free end portions of the array of support posts of each said structure to the slow wave circuit, and affixing each of the metallic support members to a body portion of a slow wave tube.
 2. The method of claim 1 wherein the step of bonding the dielectric structures to the respective metallic members includes the step of metallizing opposite sides of each of the dielectric structures and bonding one of the metallized sides of each of the dielectric structures to one of the respective metallic members.
 3. The method of claim 1 wherein the step of affixing each of the metallic members to the body portion comprises the step of bonding each of the metallic members to the body portion.
 4. In a method for fabricating a dielectric post supported slow wave circuit the steps of, bonding a plurality of dielectric support structures to the slow wave circuit for supporting the slow wave circuit, transversely slotting each of the dielectric support structures to define a plurality of arrays of dielectric support posts bonded at one end to the slow wave circuit and extending away transversely therefrom to free end portions, and bonding the free end portions of each of the arrays of dielectric support posts to a separate metallic support structure extending lengthwise of the slow wave circuit.
 5. The method of claim 4 wherein each of said metallic support structures is a metallic strip and including the step of bonding each of the elongated metallic strips to the body portion of the slow wave tube structure.
 6. In a method for fabricating a dielectric supported slow wave circuit, the steps of, slotting an elongated dielectric support structure transversely to a longitudinal side thereof to form a series of posts interconnected by spline portions, bonding said side of the dielectric support structure to an elongated metallic support member, removing the spline portions from the dielectric support structure tO define separate ones of said posts bonded at the root portions thereof to the metallic support member and extending in parallel relationship to free end portions terminating along a line parallel to said longitudinal side, while the free end portions remain along said line bonding the free end portions of said separate posts to spaced portions of the slow wave circuit, and bonding the metallic support member to a body portion of a slow wave tube to support the slow wave circuit in the slow wave tube. 